Process control systems typically use a fluid supply such as compressed air or gas to operate pneumatic process control components within the process control system. Process control systems operating in remote locations are also known to use the process media to operate components such as the pneumatic instruments and pneumatic devices such as control valve actuators. The pneumatic supply fluid is consumed during operation (i.e. a portion of the supply gas is exhausted during operation and is not captured or recycled). Depending on the process being controlled, this expenditure of supply gases can be problematic and expensive in certain instances. For example, in the natural gas industry, some pneumatic instruments operate by using the natural gas as a pneumatic supply source. Thus, the loss of high value fluids like natural gas can provide significant economic motivation to operators to detect and accurately measure leakage and, where possible, limit the consumption or bleeding of natural gas. The environmental impact of natural gas leakage and potential regulatory penalties for exceeding limits on levels of consumption or emission of natural gas create additional incentives to measure and limit those emissions. Additionally, operators may seek to measure the quantity of natural gas consumed to quantify payments for mineral rights or to facilitate tuning of the process control equipment in order to better optimize fluid consumption during instrument operation.
The total amount of supply fluid or gas used to operate a process control system may be divided into two distinct categories: supply fluid required to work the pneumatic control devices such as a control valve and supply fluid consumed or expended to operate the pneumatic control instrument. For example, in a process control system comprising a control valve and a level controller, there is an amount of supply gas that is used to actuate or move the control valve and an amount of supply gas that is consumed during operation of the level controller to generate the pneumatic signal to actuate the control valve. Typically, these two values are not distinctly identifiable, and therefore, the supply gas consumption of the process control system is approximated. As such, conventional methods do not provide an accurate estimate of the total supply gas consumption of a particular device in the system.
One conventional method of measuring supply gas consumption within a process control system is to route a pneumatic instrument's exhaust gas through a flow meter. This traditional flow measurement technique can be very inaccurate when the exhaust flow is intermittent or sporadic because the bandwidth or response time of conventional flow meter is too slow to record the deviations in flow. Additionally, the exhausted gas includes the amount of supply fluid expended by the pneumatic instrument and the amount of supply gas used to operate the valve making the individual discrimination between these two quantities very difficult. It would therefore be desirable for there to be a reliable system and method to accurately measure supply gas consumption of a particular process control instrument, or other device, within a process control system.